Here’s an article from a historian specialized in the history of the Crusades.

Excerpt:

For starters, the Crusades to the East were in every way defensive wars. They were a direct response to Muslim aggression—an attempt to turn back or defend against Muslim conquests of Christian lands.

Christians in the eleventh century were not paranoid fanatics. Muslims really were gunning for them. While Muslims can be peaceful, Islam was born in war and grew the same way. From the time of Mohammed, the means of Muslim expansion was always the sword. Muslim thought divides the world into two spheres, the Abode of Islam and the Abode of War. Christianity—and for that matter any other non-Muslim religion—has no abode. Christians and Jews can be tolerated within a Muslim state under Muslim rule. But, in traditional Islam, Christian and Jewish states must be destroyed and their lands conquered. When Mohammed was waging war against Mecca in the seventh century, Christianity was the dominant religion of power and wealth. As the faith of the Roman Empire, it spanned the entire Mediterranean, including the Middle East, where it was born. The Christian world, therefore, was a prime target for the earliest caliphs, and it would remain so for Muslim leaders for the next thousand years.

With enormous energy, the warriors of Islam struck out against the Christians shortly after Mohammed’s death. They were extremely successful. Palestine, Syria, and Egypt—once the most heavily Christian areas in the world—quickly succumbed. By the eighth century, Muslim armies had conquered all of Christian North Africa and Spain. In the eleventh century, the Seljuk Turks conquered Asia Minor (modern Turkey), which had been Christian since the time of St. Paul. The old Roman Empire, known to modern historians as the Byzantine Empire, was reduced to little more than Greece. In desperation, the emperor in Constantinople sent word to the Christians of western Europe asking them to aid their brothers and sisters in the East.

That is what gave birth to the Crusades. They were not the brainchild of an ambitious pope or rapacious knights but a response to more than four centuries of conquests in which Muslims had already captured two-thirds of the old Christian world. At some point, Christianity as a faith and a culture had to defend itself or be subsumed by Islam. The Crusades were that defense.

Pope Urban II called upon the knights of Christendom to push back the conquests of Islam at the Council of Clermont in 1095. The response was tremendous. Many thousands of warriors took the vow of the cross and prepared for war. Why did they do it? The answer to that question has been badly misunderstood. In the wake of the Enlightenment, it was usually asserted that Crusaders were merely lacklands and ne’er-do-wells who took advantage of an opportunity to rob and pillage in a faraway land. The Crusaders’ expressed sentiments of piety, self-sacrifice, and love for God were obviously not to be taken seriously. They were only a front for darker designs.

During the past two decades, computer-assisted charter studies have demolished that contrivance. Scholars have discovered that crusading knights were generally wealthy men with plenty of their own land in Europe. Nevertheless, they willingly gave up everything to undertake the holy mission. Crusading was not cheap. Even wealthy lords could easily impoverish themselves and their families by joining a Crusade. They did so not because they expected material wealth (which many of them had already) but because they hoped to store up treasure where rust and moth could not corrupt. They were keenly aware of their sinfulness and eager to undertake the hardships of the Crusade as a penitential act of charity and love. Europe is littered with thousands of medieval charters attesting to these sentiments, charters in which these men still speak to us today if we will listen. Of course, they were not opposed to capturing booty if it could be had. But the truth is that the Crusades were notoriously bad for plunder. A few people got rich, but the vast majority returned with nothing.

Since this question comes up in apologetics, and even William Lane Craig screws it up by calling the Crusades evil, I thought it might be a good idea for us to have some background so that we would be able to set the record straight if it’s called into question. It’s important to know this because a lot of people appeal to the Crusades to take shots at Christianity and introduce a kind of moral equivalence that excuses real wars of aggression and real terrorism.

The article also includes some of the real mistakes made by some of the Crusaders, so be ready to own up to those.